The present invention relates to a method for assembling packages of semi-conductor elements. Currently, packages of semi-conductor elements are classified into two general types, plastic packages and ceramic packages. Different processes are used to assemble each type of package. To help better understand the current art in the assembly of packages of semi-conductor elements, each process will be explained in detail below.
The first process to be described is that for assembling a plastic package. FIGS. 1a to 1f help describe this process, while FIG. 1g shows the finished plastic package.
Wafer 1 in FIG. 1a, which has been prepared to receive electrodes by a diffusion treatment, is subjected to a dicing process and divided into chips 2, also known as "dies." The dicing process may be accomplished by several means, including a chemical method using acetic acid or fluoroacetic acid, and a scribing method using a diamond cutter.
After the dicing process, die bonding is carried out, so as to attach a chip 2 to a paddle 4 of a previously prepared lead frame 3. This is shown in FIG. 1b. Here, the die 2 may also be called a "pellet", while the die bonding process may be called "pellet bonding" or "pellet mount". The paddle 4 is also called a "stem".
In the drawings, the lead frame 3 also comprises locking holes 5, lead fixing means or dambers 6, side rails 7, support bars 8 and leads 9. Each of the leads 9 includes an inner lead 9a and an outer lead 9b.
To accomplish the die bonding, an eutectic alloy process may be used. To do so, a gold-antimony alloy is thinly coated on paddle 4. With the die 2 then seated on the coated gold-antimony alloy layer, paddle 4 is heated. Through this heating, the gold-antimony alloy is eutectic-welded into the silicon material of the die 2. The heating temperature may be as high as 300.degree. C. to 400.degree. C., depending upon the kind of solder materials used. In order to avoid the oxidation of die 2 or paddle 4 under this high temperature, the heating is generally carried out in an atmosphere of inert gas such as nitrogen.
Other die bonding processes may be used, including use of a conductive epoxy based adhesive, a soldering process using a conventional Pb-Sn solder, and a glass soldering process. In the glass process, soldering glass is disposed on a substrate and melted at a temperature of about 500.degree. C. to 600.degree. C. A ceramic die pack is then pressure bonded on the melted soldering glass.
After the die bonding, a wire bonding process is performed, as shown in FIG. 1c. The wire bonding process connects bonding pads 10 of the die 2 with inner leads 9a of the lead frame 3 by means of wires 11. The wires are usually made of aluminum or gold. To accomplish the wire bonding process, one may use a thermal pressure bonding process, an ultrasonic process, a soldering process, a laser process or an electron beam process. The thermal pressure bonding process and the ultrasonic process are preferred in actual practice.
The above description relates to an in-line process of packages. Now, a back-end process will be described.
The lead frame 3, to which the die 2 and wires 11 have been bonded, is positioned in a die mold 12, as shown in FIG. 1d. Next, an epoxy molding compound is charged into the mold 12 and die molded. As shown in FIG. 1e, a trimming process is then performed to cut the lead fixing means or dambers 6, which were formed for the purpose of maintaining a uniform space between adjacent leads 9 of the lead frame 3. Thereafter, a forming process is carried out, so as to form the outer leads 9b into a preferred shape. After the forming process, the outer leads 9b may have a gull-wing shape, as shown in FIG. 1f, or a J-bend shape. The completed plastic package is shown in FIG. 1g.
A ceramic package is assembled in a similar manner. Ceramic packages are mainly classified into CERDIP (CERAMIC + DIP) type packages and multilayer type packages. Herein, the process for assembling a multilayer type ceramic package will be described. FIGS. 2a through 2e will be used to help illustrate this procedure, while FIG. 2f shows the finished ceramic package.
First, several sheets are prepared by using powder obtained by mixing an Al.sub.2 O.sub.3 compound with certain additives. On respective sheets, patterns are formed which are to be used in respective layers of the package. In the forming of these patterns, metal pads of leads which are to be wirebonded with the bonding pads of the die are also formed. Together with a lead frame previously prepared, the sheets are stacked in turn and then subjected to sintering so as to form a package 13 of the desired shape as shown in FIG. 2a. The ceramic package 13 takes its shape through the use of three layers, that is a bottom layer 14, a middle 15 and a top layer 16. Of course, the ceramic package 13 may have more layers.
Subsequent processes are the same as those in assembling the plastic package. That is, die bonding is carried out, as shown in FIG. 2b, to attach a die (or a chip) 17 on a paddle of the lead frame (not shown). Then, wires 18 are bonded to the die and each corresponding lead, so as to connect them as shown in FIG. 2c. In order to cover the open portion of the package 13, a glass layer 19 is then formed, as shown in FIG. 2d.
Unless the obtained package is used for manufacturing an optical element, a metal layer may be formed in place of the glass layer 19. However, ceramic packages are mainly used in the manufacture of CCDs (Charge Coupled Devices) adapted to operate by receiving light, so glass layers are the norm.
Thereafter, leads 20 are attached to predetermined positions on both sides of the package 13, as shown in FIG. 2e.
FIG. 2f reveals the structure of the finally obtained step-shaped package.
The above-mentioned conventional methods of assembling packages for semi-conductor elements, while capable of creating useable packages, have the following disadvantages.
First, although the manufacturing cost of plastic packages is inexpensive by virtue of using cheap materials in their manufacture, the process itself is complex. In addition, by performing a molding process after the wire bonding process, movement of wires may occur. Consequently, the rate of poor products is increased.
Second, although ceramic packages are advantageously used in products requiring a high degree of accuracy, since the die bonding and wire bonding are carried out after the formation of packages, the cost of manufacturing ceramic packages is quite high.